1. Field of the Invention
This invention relates to a hetero-junction bipolar transistor (HBT) and a manufacturing process of the hetero-junction bipolar transistor.
2. Related Prior Art
The HBT has an emitter with a greater band gap than that of a base. Because of its high injection efficiency for the emitter, the HBT shows a large current gain and an excellent high frequency performance, such as a current switching characteristic.
It is necessary for further improving of the high frequency performance of the HBT to reduce both the base resistance rbb′ and the base-collector capacitance Cbc. To decrease the base resistance rbb′, it is effective to increase the carrier concentration and the thickness of the base layer. Increasing of the thickness of the base layer decreases not only the resistance of the layer itself but also the contact resistance between the base layer and the base electrode. On the other hand, to decrease the base-collector capacitance Cbc, it is necessary to decrease the carrier concentration of the base layer and the collector layer, which expands a depletion layer formed at the interface between the base layer and the collector layer. The expanded depletion layer isolates the holes in the base layer and the electrons in the collector layer to each other so as to weaken a coulomb interaction therebetween, thereby reducing the capacitance.
However, to increase the base layer results in the inferior current gain. Further, since the relation of the carrier concentration of the base layer and that of the collector layer is contradictory, it will be hard to find a consistent condition thereof. Therefore, an object of the present invention is to provide an improved structure in which both the base resistance and the base-collector capacitance are decreased, and to provide a method for manufacturing thereof.